1. Field of the Invention
This invention relates to a process for producing a toner to be used for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, and the like. More particularly, this invention relates to a process for producing a toner which is suited for hot-roller fixing.
2. Description of the Prior Art
As described in U.S. Pat. Nos. 2,297,691, 3,666,363, and 4,071,361 and in other documents, various electrophotographic processes have so far been known. Electrophotographic processes, however, generally comprise forming an electrostatic latent image on a photosensitive member by a suitable method, developing the latent image with a toner, and if necessary, transferring the toner image onto an image receiving member such as paper or the like, followed by fixing the transferred image with heat, pressure, or a solvent vapor, to obtain a duplicate of the latent image.
As regards the last step of the above process, i.e. the step of fixing a toner image on paper or the like, various techniques and devices have been developed. The most common technique thereof practiced today is the hot press method by a heated roller. In this hot press method, a toner image is fixed by passing the toner-image-bearing sheet over a heated roller, the surface of which has a good toner-releasing property, so that the toner-image-bearing surface will contact the roller surface under pressure. Since the toner image is brought into contact with the heated roller surface under pressure, this method is excellent in heat efficiency and permits quick fixing, thus being applicable very effectively to high-speed electrophotographic copying machines.
According to this method, however, because of the contact of the toner in a molten state with a heated roller surface under pressure, a so-called offset phenomenon is liable to occur, that is, the toner image often partially adheres or transfers to the heated roller surface and the adhered toner in turn transfers to the incoming fixing medium, e.g. the toner image-bearing paper subsequently subjected to fixing, thus staining it. It is therefore one of the essential requirements for the hot roller fixing technique to prevent the toner from adhering to the heated fixing roller surface.
This offset is apt to occur particularly when a low molecular weight resin is used in the toner as the binder resin. It is conceivable in consequence that the offset phenomenon is possibly prevented by using a crosslinked resin. However, investigations by the present inventors revealed that the use of a simply crosslinked resin does not always give a good result. In particular, with an increase in the degree of crosslinking, the required fixing temperature rises and in the case of magnetic toner the anti-offset property becomes inferior.
Practice has been made with a view to prevent the toner from adhering to the fixing roller surface; for instance, the roller surface is coated with a material excellent in toner-releasing property, such as a silicone rubber or fluoroplastic and this coating is further covered with a thin film of liquid, such as a silicone oil, excellent in releasing property, so as to prevent the offset phenomenon and the deterioration of the coating. Although very effective in preventing the offset phenomenon, this practice has problems such that the offset-preventing liquid evaporates off upon heating and smells unpleasant for the operators and that the fixation apparatus is complicated because of a device for supplying the offset-preventing liquid. Accordingly, a desirable approach to the prevention of the offset phenomenon is not to supply such a releasing liquid but to develop a toner which is fixable over a wide temperature range and excellent in anti-offset property.
While it is a matter of course that the toner is required to be good in not only fixability but also the anti-blocking property, developing ability, transferability, cleanability, etc. The existing toners have one or more drawbacks like the following. Many toners fusible at relatively low temperatures tend to cake or agglomerate during storage or standing in a copying machine. Most toners become worse in triboelectric property and flow property with the variation of environmental humidity. At repeated and continuous image development, most toners, carriers and photoconductor surfaces usually deteriorate due to the mutual collision and adhesion. This results in variation of developed image density and increased background optical density, thus degrading image quality. Moreover, if amounts of most toners adhering to the latent-image-bearing photosensitive member surface are increased for the purpose of increasing developed image density, the background optical density usually increases, that is, the so-called fogging occurs. Such being the case, a toner is desired which is excellent in those various properties as well as suitable for fixation with a hot roller.